What is a nonscalar curvature singularity, in the context of "the https://en.wikipedia.org/w/index.php?title=Wave_of_death&action=edit&redlink=1 [Broken] is a gravitational plane wave exhibiting a strong nonscalar null https://en.wikipedia.org/w/index.php?title=Curvature_singularity&action=edit&redlink=1 [Broken], which propagates through an initially flat spacetime". The quote is from wiki, https://en.wikipedia.org/w/index.php?title=Pp-wave_spacetime&oldid=666287121#Examples. There is a rather old post on the topic of "the wave of death", https://www.physicsforums.com/threads/wave-of-death.93654/, on PF which was quite interesting, but I'm not quite following the details as I'm not sure what a nonscalar curvature singularity is. My best guess is that because scalar invariants, such as the Ricci scalar, all vanish for the pp wave class of space-times, one looks to higher-order tensors with "strong" components. What qualifies a component as being "strong" or "singular" is unclear to me at this point, I would speculate perhaps a tensor with dirac-delta function comonents would qualify?